jeffythedragonslayer wrote:
Ok thanks, so it's like musical chairs trying to cram more bits in there.
Correct.
The reason SR (aka P, which it is hardly ever called in machine language monitors) is 8-bits wide has to do with the 65C02 emulation mode and Apple's influence (meddling, some might say) in the 65C816's design process. Already having a 16-bit ALU, it would have been trivial to give the 816 a 16-bit SR and thus expose the e bit as a separate entity. However, that would have meant SR would always be a 16-bit register, even in emulation mode. Ergo emulation mode would not have been (almost-true) CO2 emulation, as the sequence of PHP - PLA, for instance, would have not worked as it would in a real 65C02.
As Garth noted, software seldom does anything with e. Most native-mode software immediately switches the 816 to native mode with the CLC - XCE sequence, and that is the only time e is touched. As the 816 comes out of reset in emulation mode, the programmer doesn't have to do anything if he wants to stay in emulation mode.